Lithium battery technology is a relatively new field and the subject of intensive research. The main battery characteristics sought to be improved by new research are size, weight, energy density, capacity, lower self discharge rates, cost and environmental safety. The goal is to simplify the fabrication techniques and improve interlayer adhesion to produce a dry cell battery that is small and light weight, has a long useful life, has greater energy density, and contains little or no toxic compounds that may enter the environment upon disposal. These batteries are useful for many applications such as power supplies for cellular phones, smart cards, calculators, portable computers, and electrical appliances.
Schmutz et al. (U.S. Pat. No. 5,470,357) address a problem with adhesion between the electrode and collector elements. Their solution is to pretreat the collector elements in which a 0.25% to 3.0% solution of a polymeric material compatible with the matrix polymer is applied to the collector foil or grid and dried to form a coated film. The resulting coated collector element is heated to make the polymer tacky. The pretreated coated collector element is further processed by applying the appropriate electrode composition to it to form an anode or a cathode. These electrode and separator elements are formulated as layers of plasticized polymeric matrix compositions that are laminated with the electrically conductive collector elements to form a unitary battery cell structure.
Gozdz et al. (U.S. Pat. No. 5,587,253) disclose a lithium ion battery with an electrolyte/separator composition comprising a polyvinylidene fluoride copolymer and a plasticizer. The crystalline structure of the polyvinylidene fluoride copolymer necessitates the introduction of plasticizers to disrupt the crystalline regions of the copolymer matrix simulating an amorphous region which leads to higher ionic conductivity. In addition, the introduction of plasticizers helps to lower the glass transition temperature of the polymer allowing it to undergo melt flow or softening during the operation of the battery. This helps to facilitate the mobilization of the ions through the membrane. Eventually, the plasticizer must be replaced with an electrolyte salt solution which contains another plasticizer combination of ethylene carbonate and dimethyl carbonate.
The battery is formed by laminating each of the composite anode and cathode compositions separately onto wire grid current collectors. Gozdz et al. further address the problem with adhesion by surface cleaning the electrode elements in a common copper bright solution, rinsing in water, air drying, dip coating in an acetone solution of the copolymer solution and drying it to a tacky state. In particular each electrode was prepared by cutting a film and overlaying it upon the dip coated grid to form an element pair. The element pair was placed between buffer sheets of abherent polyethylene terephthalate and was then passed through a laminating station. An electrode/collector pair was laminated with an interposed electrolyte separator membrane. In order to activate the battery, the laminated battery structure was extracted of a substantial amount of the plasticizer comprising the polymer matrices of the laminated layers, particularly the separator/electrolyte. The extracted battery structure was then activated in preparation for charge/discharge cycle testing by immersion, under a substantially moisture-free atmosphere. During immersion, the battery imbibed an amount of a 1M electrolyte solution of LiPF.sub.6 in 50:50 ethylene carbonate (EC):dimethyl carbonate (DMC) for about 20 minutes during which the battery imbibed an amount of solution which substantially replaced the extracted plasticizer with the EC/DMC solution.
Skotheim et al. (U.S. Pat. No. 5,601,947) disclose "gel-type" solid electrolytes that consist of a high molecular weight polymer matrix into which is dissolved an electrolyte salt, then subsequently swollen with a low molecular weight liquid (propylene carbonate, ethylene carbonate, glymes, low molecular weight polysiloxanes, and mixtures thereof) which effectively acts as a plasticizer for the salt-polymer matrix. Useful gel-type electrolytes include sulfonated polyimides which have been swollen. The introduction of these plasticizers affects the dimensional stability of the material in that they have a tendency to leach out of the material causing it to return to a brittle and inflexible state. This affects the ionic mobility of the system and causes the adhesion to fail.
Whang (U.S. Pat. No. 5,407,593) teaches that the main path for ion transportation in a polymer electrolyte is via the amorphous region of a polymer matrix. Thus, the ionic conductivity of a polymeric electrolyte can be increased by diminishing the crystalline region and increasing the amorphous region of the polymer matrix. The methods frequently used to achieve this are: (1) preparing a new polymer such as copolymer or polymer with network structure; (2) adding non-soluble additives to improve the electrolytic property; and (3) adding soluble additives to provide a new path for ionic conductivity. Polymers having high-dielectric constants are good matrices for preparing polymeric electrolytes. However, because they have high glass transition temperatures or high degrees of crystallinity they do not result in desirable polymeric electrolytes. To remedy this, Whang discloses a polymeric electrolyte containing no volatile components. This assures that no change in conductivity and composition occurs due to the volatilization of some compounds contained therein. Thus, the conductivity is kept constant. The polymeric electrolytes of his invention include a polar polymer matrix, a dissociable salt, and a plasticizer of polyether or polyester oligomer with terminal groups halogenated.
Fujimoto et al. (U.S. Pat. No. 5,468,571) disclose a secondary battery with a negative electrode comprising a carbon powder, particles constituting the powder being consolidated with a polyimide binder. The polyimide may be either a thermosetting polyimide or a thermoplastic polyimide, the former including both condensation type and addition type. A representative example of condensation type polyimide resins is one obtained by heat curing (dehydration condensation reaction) a solution of a polyamide acid (a polyimide intermediate) in N-methyl-2-pyrrolidone. The polyamide acid in turn being obtained by reacting an aromatic diamine with an aromatic tetracarboxylic acid anhydride. The heat curing is preferably conducted at a temperature of at least 350 degrees C. for at least two hours to complete the dehydration condensation reaction. They note that if the polyimide intermediate with which the dehydration condensation reaction has not been completed, remains in the negative electrode after the heat curing, it may, when the battery temperature becomes abnormally high, condense to release water which should react vigorously with lithium. Addition type polyimides also require heat curing.
An object of the present invention is to provide a polyimide battery that is based on a soluble, amorphous, thermoplastic polyimide.
Another object is to provide a polyimide electrolyte that does not require swelling or introduction of plasticizers.
Another object is to provide a polyimide battery that exhibits excellent interlayer adhesion.
Another object is to provide a polyimide battery that is capable of dissolving a large amount of lithium salt.
Another object is to provide a polyimide battery that is environmentally safe.
Another object is to provide a polyimide battery that exhibits high ionic conductivity with insubstantial change over a range of temperatures and pressures.
Another object is to provide a process for preparing a polyimide battery.
Another object is to provide a process for preparing a polyimide battery that does not require pre-treatment of the current collectors.
Another object is to provide a process for preparing a polyimide battery that does not require curing of the polyimide.
Another object is to provide a process for preparing a polyimide battery that does not require heating the polyimide above its glass transition temperature.
Another object is to provide a process for preparing a polyimide battery that does not require high temperature and high pressure to form the battery.
Another object is to provide a flexible polyimide battery.